Has anyone any ideas on using a Transformer (1kVA) the wrong way round. The transformer has a 230VAC primary and a 110VAC secondary. Would it be OK to use the secondary as the input(110VAC) and put the load on the primary - a 230VAC motor. Only need about 250Watts.
Will it work?
Michael.
Sure it will.
The secondary just becomes the primary, and the primary the secondary.
I'm not sure about the overcurrent protection required in your part of the World, but it will work properly
Normally I would say yes, if it were 240/480 volt. But I am not sure with 120 volt. You only have one CCC and a neutral. But I think you are okay. Protect XFMR on line side as current is less.
I have never back fed a 120/240 volt XFMR, so I am not sure about the neutral.
J Valdez:
At "110" Volts you will have 2 current carrying conductors...both the ungrounded conductor and the grounded conductor are current carrying.
The current on the line side will be MORE than the current on the load side.
The OP never said this is a 120/240 transformer, rather a 230 to 110 transformer.
We used a "backward" transformer in our shop for many years. I had a 240v blower motor out of a tape drive we used for shop air on the 120v they supplied us using a convenience outlet transformer, usually providing 120 out of the 240 in the computer room.
electure....Thanks for the clarification. I should have counted the nuetral as a CCC.....John
Wouldn't you need to ground one of the secondary (230-volt)conductors?
No. Use a seperate BONDING conductor to "ground" the secondary enclosure.
Note that the 120 primary will draw 2+ the amps of the secondary load.
Ohms Law.....Obey it. God doesn't give tickets.
You would be permitted to ground one of the conductors.
Would it be advisable to ground one or not?
Opinions, please?
We discussed this before. If the transformer is part of the building wiring you need to ground the secondary, with few exceptions. If it is a cord and plug connected appliance you don't. (that was where I went wronmg the last time it was discussed) It is very common that a lab work bench supply is flaoting but this is a cord and plug cionnected appliance, not building wiring. We had the same situation with our compressor at the shop. The whole assembly was behind a 5-15 plug.
I'm thinking that since it does not have a center tap, and grounding a line conductor will not keep the voltage to ground below 150 volts per 250.20 (B) (1), the secondary MAY be grounded according to 250.21(4) but is not required to be.
I'd like opinions on the advantages and disadvantages of grounding it or not?
I see absolutely no good reason to bond one leg. It only adds a level of danger if you get between line and ground.
Exactly Greg,
Isolating transformers have worked without the secondary side of them grounded for years.
As long as both of the secondary wires are fused, I can't really see any issues with a set-up like this.
I would ground the transformer body though, just to be on the safe side.
I understand that grounding one leg will raise the voltage to ground to 230 volts, similar to the way a corner grounded 480 delta raises the voltage to ground to 480.
However, I thought that grounding a secondary conductor helps keep the voltage stable. (Scott 35?)
I have no opinion either way. Just looking to open a discussion.
Trumpy is right, you should always ground any non current carrying metal in just about everything electrical. That is why we have EGCs. I understand why you might need to ground one leg of a service to stablize voltages but in a transformer secondary feeding a single piece of equipment or a small number of local loads I am not sure where any instability would come from.
If you have issues with EMI or EMC, grounding one side or the center-tap of the secondary, will provide a place to shunt the noise to. Many EMI filters shunt noise to the chassis. If there is no chassis path to "earth" they can use the "grounded" conductor as a back up path.
Larry C
Bigger question. Why not to pursue this path in theory?
Grounding line on secondary along with grounding line on system will lead to high potentials thru out and or short circuits on EG's
Not a good practice at all.
When wired "backwards" small transformers (smaller than 3kVA singel phase) will exhibit worse than normal output voltage regulation and higher than normal inrush current. In either case these are not normally deal breakers just things you should be aware of.
The voltage regulation falls off because the "H" windings usually incorporate additional turns to compensate for the inherently poor regulation (well relatively so compared to bigger units) of small transformers. Now that the extra windings are on the output side they offer little help.
The inrush current can be up to 14x due to the location of the H and X windings in relation to the core. This is usually not a problem for transformers with less than 9A rated load on the X windings.
An isolation transformer is a separately derived system and must be grounded per article 250. It makes no difference if there is a grounded conductor on both the transformer primary and secondary, you must follow correct grounding and bonding practices.
Yes JBD, I agree , no deal breaker. But.
Not something to be taken farther than control Xfrmr. Yes?
An isolation transformer is a separately derived system and must be grounded per article 250. It makes no difference if there is a grounded conductor on both the transformer primary and secondary, you must follow correct grounding and bonding practices.
But wouldn't having an isolating transformer with both of the windings grounded be a waste of time, considering that the ground connection would be common to both windings?.
I was always under the impression that a double wound transformer was made so that there would be no reference to the primary winding by the secondary winding, hence the term isolating transformer.
That is true mike and we do it all the time (isolated secondaries). Look at the transformer for a pool light where safety is job #1. It is isolated from ground.
It comes down to what that transformer is being used for. On a service bench, we'll use a 1:1 isolation xfmr with the secondary floating simply to remove the ground reference from the radio/TV being worked on. It reduces risk of shock due to accidental contact and permits grounded test equipment to be connected to "live chassis" apparatus for testing and alignment.
Look at the British bathroom shaver outlet for a similar 1:1 xfmr example. The secondary is left deliberately ungrounded to reduce possible shock risk. They key here is that the xfmr is right at the outlet itself, thus resulting in a short and easily controllable section of ungrounded circuit, just as with test bench example.
If you use those ungrounded secondaries to feed into much more extensive building wiring, capacitive effects at 50/60Hz will start to become much more apparent. Grab hold of just one side of a U.K. shaver outlet and you won't feel much (if anything). Do that with a wiring system running over a much larger area and the stray capacitance could be enough to deliver a significant punch.
Yes JBD, I agree , no deal breaker. But.
Not something to be taken farther than control Xfrmr. Yes?
Actually no.
I would almost never recommend it for a control transformer, but I have very few reservations about doing it with a large power transfromer. The control transfromer will probably have lousy voltage regulation. But I always check with the transformer manufacturer first.
JBD, you are sounding as if you are on another topic, to me anyway.
The code allows little in regards to isolation transformers, they shall not have grounded secondaries.
You could ground a control xfmr but there are many applications were it would be better to not do so.
Your postion here is not consistent with standard prevention of hazards
517.160 Isolated Power Systems
Also in regards to control xfmr's, there are a boat load of reasons to NOT ground control xfmr's in obvious industrial applications.
Ive wired operating rooms were "isolation" from ground by secondaries is a "shall" issue by all equipment and installation and code.
Furthermore you stated that you must ground all xfmr;s as per 250. I dont agree with that as spec'd above
There is a place in the electrical contruction idustry and elsewhere, ARTICLE 517 Health Care Facilities for not grounding xfrm's secondaries.
This OP was about back feeding a xfmr and evolved to grounding line on the secondary of said back fed xfrm.
JBD, can you elaborate on why you have no problem doing this on large xfmr's and your reasons why NOT to do it on control and isolation xfmr's as the NEC clearly requires.
In my tours of duty in industrial maintenance we requently "floated" the neutral on control xfmr's. It was backed up by the "engineering" staff,. And the NEC.
.
Yes JBD, I agree , no deal breaker. But.
Not something to be taken farther than control Xfrmr. Yes?
Actually no.
I would almost never recommend it for a control transformer, but I have very few reservations about doing it with a large power transfromer. The control transfromer will probably have lousy voltage regulation. But I always check with the transformer manufacturer first.
I guess I am, sorry.
I was talking about not reversing connecting control transfromer, I didn't realize we had switched to ground/bond versus float.
Always ground transformer secondaries per 250. I cannot think of any good general reason to float a secondary.
An isolation transformer is a separately derived system and must be grounded per article 250. It makes no difference if there is a grounded conductor on both the transformer primary and secondary, you must follow correct grounding and bonding practices.
But wouldn't having an isolating transformer with both of the windings grounded be a waste of time, considering that the ground connection would be common to both windings?.
I was always under the impression that a double wound transformer was made so that there would be no reference to the primary winding by the secondary winding, hence the term isolating transformer.
Trumpy,
This happens often for example, a 277V primary and a grounded 120V secondary in a lighting control panel or a 120V primary down to a 24V secondary in a furnce system.
Hi All,
I want to thank everyone for their input on this topic.
One last question:
Does it break any US electrical wiring codes to use a transformer wrong way round, i.e. line connected to primary?
TiA,
Michael.
The line is always connected to the primary. In general the primary is the high voltage winding, but there are no Code issues with treating the low voltage winding as the primary.
You'll get crummy voltage regulation and various other potential issues, but no Code problems.
The distribution system uses step up transformers at the source end so they must be able to handle the voltage regulations problems.
The distribution system uses step up transformers at the source end so they must be able to handle the voltage regulations problems.
The voltage regulation issue only occurs when a transformer is reverse connected.
May want to check with the manufacturer. Can have compensated windings that would cause lower voltage output when reversed. Don't check for it, but noticed one stated it could be reversed on the nameplate information.